Contract half of the pair #8245 started, now that #8245 is live everywhere: since that release a bare body mention of an issue key writes no link row, and no query reads or writes the column.
Migration 468 deletes any reference_only rows an instance of the older release could still have written during #8245's rollout — once the column is gone such a row would be indistinguishable from a real link, visible in the issue's PR list and blocking auto-advance while its PR is open — and then drops the column from issue_pull_request and issue_vcs_pull_request. sqlc drops the two phantom struct fields with it.
The down migration re-adds the column so a pre-#8245 instance can run against a rolled-back schema; the deleted rows do not come back and no read path has returned them since #8245.
